jetrainor

Have a line on a low mileage 1990 motor and have requested a leakdown test however i have no idea what it actually is nor how to interpret the numbers that come out of the test. What sort of numbers would represent an engine in good condition?

87 euro auto

IcemanG17

Jet
Okay I'm not an engine guy, but I understand it like this....it measures the amount of pressure that leaks out of the cylinder...the less the better....typically it is recorded by % per cylinder....even a new engine will have some leakdown....what to look for is similar % cylinder to cylinder....if several are in the 3-5-4-5 range and one is at 15....there might be a problem.

mulik51

Interesting post!
I want to do a leakdown before putting the engine in. So I went to the Machine Shop which I did my heads at(they specialize on 944), and asked them for how much would they do it. The guy there told me that I can go down to PepBoys and get a leakdown tester. I said that this is NOT a compression test, and on the pictures of leakdowns, I saw some complicated stuff, but the guy insisted that I can do it myself. So, any advice? Can I perform it myself? Or I need to carry my engine over?

Thanks a lot,
Klim

P.S. Yeah, I think the numbers are read as percentages of leakage. You put the piston to TDC, and pump air in it, measure the pressure. Then wait like 60sec, and measure again(on the pics I saw there were to pressure gauges). Then figure out the percentage. I heard that if you have 2% or less, it is considered perfect. Also, if hear air coming out from intake-intake valve are faulty, if you hear it coming out of the headers-exaust valve faulty. If you hear it coming out at the T-Housing-head gasket. And, if you have it come out at the hole where you put the oil in-it is you rings. I think that should be correct

dr bob

You can do it yourself. There's a tool/fixyure with a couple of pressure gauges, a regulator, and a calibrated orifice tube with a .100 hole an inch long inside IIRC. You'll need an air compressor that can maintain 100 PSI for the test, hopefully at a fairly low volume but you never know.

Roatte the engine to TDC on the cylinder under test. Plug the business end of the tester into the spark plug hole. Set inlet regulator to 100 PSI with air hose connected, and read the pressure from the other gauge. The difference between the two gauges is your leakdown percentage. Repeat for all cylinders.

If you find a cylinder with a lot of leakage, you can sometimes listen at the intake, then the exhaust, and the oil filler for audible evidens of air passing through. These relate to intake valve leaking, exhaust valve leaking, piston/rings leaking, respectively.


Does this sound like someting you can do at home? Sure! You'll need the tool, which is available from sevaral sources including places like Summit Racing and most airplane engine places. You also need that air compressor with the regulator.

mulik51

Thanks a lot, I will check out the automotive shops around here,


Klim

jetrainor

So, what represents good numbers?.....if 2% is excellent, is 4 or 5% acceptable....also, would it make a difference if the engine was warm or cold?....the motor i'm considering buying was pulled from the car 8 months ago so obviously the leak down test will be done on a 'non running' engine. The parts shop tells me a leak down test is more thourough than a compression test.

87 euro auto

mulik51

It is more thorough then compression test becouse it tells you where you have a leakage.

Actually, if you go to "Advanced Search" and put "leak down" and search titles you should come up with the info on the good numbers(that what I did like 2 moths ago)

Klim

WICruiser

Leak down is significantly better than a compression test because it measures leakage past the critical sealing elements under a static pressure; piston rings and valves. A compression test is similar except that leakage past the rings and valves can be slow relative to piston speed creating pressure under dynamic conditions. I agree with the above that you are looking for consistency from cylinder to cylinder, not sure what numbers to expect on this engine.

The other advantage you have is with the engine out you can rotate it to TDC on each piston, run the test, and move on to the next. Cranking the engine fast enough on a stand to measure compression can be "interesting".